A seat belt assembly for a motor vehicle typically has a seat belt retractor that retracts belt webbing onto a spool in a housing. When the belt webbing is drawn or protracted from this housing, the spool winds a retraction spring, which later causes the spool to rotate in an opposite direction to retract the unused portion of the belt webbing back onto the spool.
In a crash, the seat belt retractor has a lock that prevents the belt webbing from extending further from the housing. The lock may be actuated by an inertial sensor, which responds to changes in vehicle acceleration that occur during the crash. When a large change in vehicle acceleration is detected, the inertial sensor triggers the lock of the seat belt retractor to lock the spool against movement and thereby secure the belt webbing in place during the crash.
The inertial sensor has a dense inertial sensor mass that moves in a sensor housing to respond to changes in acceleration. This inertial sensor mass is typically made of metal because of its high density. In this way, the inertial sensor mass may be kept small and still retain its sensitivity. However, during normal vehicle operation, the free movement of the metal mass within the sensor housing causes rattling that may be unpleasant to hear. A need therefore exists for a seat belt retractor that reduces the undesirable noise associated with movement of the inertial sensor mass.
A prior art inertial mass sensor 154 employing a metal outer mass 160 with an elastomeric center 162, conical upper top 194 and flat bottom 190 is described in U.S. Pat. No. 6,164,581 entitled “Low Noise Self Compensating Vehicle Sensor and Retractor”. This prior art sensor 154 as shown in FIG. 10 provided some noise reduction at the upper and lower contact points. A primary drawback of this prior art sensor was the tilt sensitivity was diminished due to an increase in friction drag on the actuator lever 140. Additionally, vibrational frequencies of the vehicle were still being transmitted through other contact points on the sensor surface and the housing 150 resulting in a pronounced noise signature. Accordingly a new approach to sound deadening is needed that can be accomplished without degrading the sensitivity of the inertial mass sensors to changes in acceleration.